The present invention relates to secondary (rechargeable) batteries which utilize electrodes comprising intercalation compounds, principally lithiated ternary transition metal oxides, such as LiMn.sub.2 O.sub.4, LiCoO.sub.2, and LiNiO.sub.2. In particular, the invention yields an improved electrolyte element which serves to more effectively separate the battery electrodes while providing a highly functional and economical electrolytic medium for the intercalation process.
Early secondary lithium battery structures typically comprised an elemental lithium metal negative electrode physically separated from a positive electrode of intercalatable compound by an electrically insulating element which served a secondary role as a means of immobilizing a fluid electrolyte for the system. Although some success has been reported in the use of a unique solid electrolyte (Meunier et al., Mat. Sci. and Eng., B3 (1989) 19-23), commercially feasible electrolyte elements have for the most part been sheets or films of porous materials, such as glass fiber filter paper or cloth (U.S. Pat. No. 4,751,159) and microporous polyethylene or polypropylene film or nonwoven cloth (Ohzuku et al., J. Electrochem. Soc., Vol. 137, No. 1, 1990) saturated with solutions of lithium compounds, typically LiClO.sub.4 or LiBF.sub.4 in propylene carbonate, diethoxyethane, or other organic solvent (U.S. Pat. No. 4,828,834).
These separator/electrolyte elements were not wholly satisfactory, however, due to their relatively great thickness, which resulted in excessive separation between electrodes, and their large pore size, which allowed a dangerous level of lithium dendrite formation at the positive electrode upon recharging. Glass fiber cloth, for example, resulted in electrode separation of about 300 micrometers while providing restrictive channeling down to only about 250 nanometers. Some improvement was provided by costly microporous polyethylene membranes of about 50 micrometer thickness, but these elements were difficult to individually maneuver into battery structures and presented the additional disadvantage of low strength and excessive thermal sensitivity.
There has since been substantial improvement in battery components, such as by the substitution of lithium intercalated compounds for hazardously reactive lithium metal. The most practical electrolytes from the standpoint of functionality, reasonable cost, and ease of handling continue, however, to be the organic solutions containing lithium ion salts. The present invention, therefore, marks a notable improvement in the fabrication of secondary lithium batteries by providing a medium for immobilizing these fluid electrolytes in a separator element of minimal thickness and pore size, thereby increasing the efficiency of electrolytic activity across the reduced interelectrode span and allowing the use of all manner of electrode material, including elemental lithium.